Mapleson used a computer spreadsheet model to predict the theoretical ideal fresh gas flow sequence at the start of low‐flow anaesthesia. The aim was to increase the end‐expired partial pressure of inhalational agent (P~E′an~) to one minimum alveolar concentration (MAC) as quickly as practicable and then to keep it constant. Ninety adult patients undergoing elective tonsillectomy under general anaesthesia were randomly allocated to one of three groups (n = 30) to receive isoflurane, sevoflurane or desflurane in oxygen. Fresh gas flow and vaporiser settings as specified by Mapleson were followed in all cases except that the maximum setting for desflurane was 18% (2.7 MAC instead of 3 MAC). Recordings of P~E′an~ were made at 1, 2, 3, 4, 5, 7, 10, 15 and 20 min. Mean values of P~E′an~ exceeded 1 MAC by 2 min in all three groups and remained above this value throughout. Each group's P~E′an~ measurements were divided by their respective 1‐MAC value. A simple two‐level model (with patients at level 2 and time at level 1), with measurements at 1 min excluded, showed that the fitted value at 2 min and the time‐weighted mean for 2–20 min for P~E′iso~ (1.042 [95% CI 0.980–1.104] and 1.044 [0.984–1.104], respectively) were not significantly different from its 1‐MAC value, whereas those of the P~E′sevo~ (1.169 [1.119–1.219] and 1.143 [1.119–1.219]) and P~E′des~ (1.305 [1.261–1.349] and 1.140 [1.098–1.182]) were significantly higher than their respective 1‐MAC values. The Mapleson concept of an initial high fresh gas flow and high vaporiser settings, followed first by reduced high fresh gas flow, as followed in this clinical study, results in P~E′an~ values close to or slightly higher than predicted in the spreadsheet model.